Various aerosol chargers are currently available to impart electric charge, of either positive or negative polarity, on airborne particles. Charged aerosol particles can be deflected in an electric field. The electric force on submicron particles can be more than a million times greater than other forces, e.g., gravitational force. Consequently, electric charging of aerosol particles followed by electric field manipulation, is a principal technique used in the measurement and control of submicron particles.
Such measurement and control of submicron particles is beneficial with regard to many applications. For example, such applications include the control of fly ash from power plants by electrostatic precipitation and in the measurement of aerosol size distributions. Further, aerosol technology has been used for the synthesis of new materials with particular interest in production of ultrafine particles such as for applications including photoimaging, magnetic tapes, filtration and electro-optical applications. To develop such technologies, techniques are needed to measure the particle size distributions, to classify the particles into monodisperse fractions, and to recover the particles from the aerosol form. In addition, particulate contaminants are major causes for yield losses in semiconductor device fabrication, particularly for submicron features. Particles smaller than the feature sizes must be controlled and measured to be able to address such contamination problems. The principle of charging particles and manipulating such particles in an electric field are used in the development of such technologies and for arriving at solutions to address such problems.
Airborne particles can be charged by various methods, for example such methods include contact (triboelectric) charging, spray charging, photoelectric charging, or by gaseous ions. Charging by gaseous ions is a preferred method for aerosol charging because such charging by gaseous ions is a repeatable process and is theoretically predictable.
Charging by gaseous ions involves producing gaseous ions of one polarity (unipolar) or of both polarities (bipolar), and allowing the ions to collide with aerosol particles due to their random thermal motion. In other words, unipolar charging is the charging of particles in a gaseous medium containing ions of one polarity and bipolar charging is the charging of particles in a gaseous medium containing ions of both polarities.
Various unipolar aerosol chargers are available and/or have been described. One currently available charger is the unipolar aerosol charger used in the Electrical Aerosol Analyzer available under the trade designation TSI Model 3030 from TSI, Inc. (MN). This charger is described in the article by Pui, et al., entitled "Unipolar Diffusion Charging of Ultrafine Aerosols" Aerosol Science and Technology, 8: 173-187 (1988). This charger involves passing ions in a stream perpendicular to the aerosol particle stream passing through the device. As the aerosol particle stream passes through the stream of unipolar ions, the aerosol particles are charged via collisions of the aerosol particles with the unipolar ions. The ion stream is directed with an electric field perpendicular to the aerosol stream.
Another similarly configured particle charger with an ion stream perpendicular to the aerosol stream is described in an article by Buscher, et al., entitled "Performance of a Unipolar `Square Wave` Diffusion Charger With Variable nt-Product," J.Aerosol.Sci, Vol. 25, No. 4, pp. 651-663 (1994). This charger uses a chopping electric field created using a square wave voltage across the charging zone to alleviate particle losses.
Yet another charger is described in an article by Romay et al., entitled "Unipolar Diffusion Charging of Aerosol Particles at Low Pressure," Aerosol Science and Technology, 15: 60-68 (1991). This charger includes an elongated tube having a voltage applied between two metallic screen electrodes. One of the screens is positioned at an aerosol inlet and the other at an exit of the charger. A radioactive source is positioned close to the inlet such that when a voltage is applied, bipolar ions fill the volume by the inlet screen and unipolar ions fill the region between the source and the exit screen electrode. The unipolar ions filling the region charge the particles with such charged particles exiting through the exit screen. An electric field is created in the ion containing region; however, unlike the previously described chargers above, the electric field is not perpendicular to and does not direct the ions perpendicularly relative to the aerosol stream passing through the charger. The electric field of this charger includes an electric field that includes fringes directed to the wall of the charger. Various other chargers have also been described. For example, another unipolar charger is described in the article by Wiedensohler et al., entitled "A novel unipolar charger for ultrafine aerosol particles with minimal particle losses," J.Aerosol.Sci Vol. 25, No. 4, pp. 639-650 (1994).
Charging of the aerosol particles becomes increasingly difficult as particle size gets smaller because the collision cross section of the particles becomes small relative to the mean-free-path of the ions. In the nanometer particle size range, e.g., 2 nm to 100 nm, such nanometer particles seldom carry more than a few elementary units of charge. For example, below 10 nm, an increasing fraction of the particles are not charged, i.e., they are electrically neutral, leaving only a small fraction of the nanometer particles being charged. Additionally, the small fraction of charged particles often are lost in the charger due to the high electric mobility of such charged nanometer particles. The currently available chargers or described chargers are not adequate to charge submicron particles, particularly not adequate for charging nanometer particles. Such chargers have charged particle losses that are undesirably large leading to only a small fraction of charged particles being allowed to exit the charger.
For example, most of the existing chargers involve passing an ion stream perpendicularly to the aerosol particle stream flowing through the charger. The charged particles resulting from collisions with the ions in the ion stream are deflected in the same direction as the flow of the ion stream. Such deflection results in significant charged particle loss due to the charged particles being deflected and captured on surfaces of the charger.
Further, Brownian diffusion causes the aerosol stream to spread as it passes through existing chargers further causing charged particles in the stream to be in close contact with charger structure. Contact with such charger structure also results in losses of charged particles.
Yet further, many existing chargers have structure or obstructions therein lying in the path of the flow of the charged particles through the apparatus that cause charged particle losses. For example, charged particle streams flowing through chargers wherein the charged particles must exit the charger by passing through an obstruction, e.g., such as a screen electrode at an outlet of the charger as described above, results in charged particle losses.
The importance of charged particles, particularly nanometer particles, for technological applications is known as described above. For example, nanostructured materials composed of nanometer particles often possess significantly enhanced mechanical, optical, electrical/magnetic properties, which are desirable for advanced engineering applications. Many of the advanced applications, e.g., tunable lasers, require depositing uniform size nanometer particles in layers. The uniformity requirement is quite stringent, often within 5% of the mean size. The particles can be made uniform by passing through a differential mobility analyzer (DMA) which classifies the airborne particles in a condenser according to the particles electrical mobility. A narrow slit is used to extract particles with nearly the same mobility. The DMA is therefore equivalent to that of a band pass filter with a narrow mobility window. By connecting a particle counter downstream of the DMA, the system can be used to measure particle size distributions which can be inferred from the electric mobility distributions.
The difficulty involved in DMA classification and measurement for particles, and in particular nanometer particles, is that only a small fraction of particles introduced into a charger are actually charged and exit the charger. A large fraction of the charged particles is typically lost within the charger, resulting in a small extrinsic charging efficiency.
For this reason and other reasons, there is a need in the art to provide high charging efficiency and low loss characteristics, i.e, a high throughput charger which overcomes the disadvantages as described above. The present invention addresses such needs and alleviates such problems described above, and other problems as will become apparent to one skilled in the art from the detailed description below.